In general, multicast and broadcast service (MBS) is a mechanism for distribution of multicast (source to multiple recipients) and broadcast (source to all recipients) data (MBS data) content across multiple base stations (BS) from a centralized server to mobile stations (MS) in a manner that may take advantage of orthogonal frequency division multiple access (OFDMA) macrodiversity. An aim of macrodiversity is to combat fading and to increase received signal strength.
In OFDMA systems, radio resources consist of subcarriers (resources in a frequency domain) and OFDMA symbols (resources in a time domain), with an OFDMA frame including several OFDMA symbols and a resource allocation of one or more subcarriers. In order to achieve macrodiversity in OFDMA systems with a configuration commonly referred to as single frequency network (SFN), whereby all BS in a MBS zone are transmitting on the same carrier frequency, MBS data in the MBS zone needs to be transmitted using the same set of subcarriers and with a time occurrence within an OFDMA frame that is the same for all BS, i.e., the MBS data transmitted by the BS occupies the same OFDMA symbol locations within the OFDMA frame as well as the same number of OFDMA symbols.
Since MBS data sent on the frequency domain in an OFDMA system is already orthogonal, the time domain portion of the requirement for macrodiversity may be achieved by synchronously transmitting the MBS data from multiple BS. By transmitting the MBS data from the multiple BS at the same time (as specified by the OFDMA symbol locations within the OFDMA frame and the number of OFDMA symbols), the energy of the MBS data transmission from each BS may be combined over the air such that the combined energy is at least equal to or greater than the energy from only a single BS. This is expressible as
            ∑              i        =        1            M        ⁢          E      i        ≥      E    X  where Xε{1, 2, 3, . . . , M} represents any one of M BS in the MBS zone, Ei represents a received energy from a BS at a MS, whereby the summation is valid if and only if the MBS data was transmitted using exactly the same subcarriers and location in time as specified via the OFDMA symbol locations in the OFDMA frame. Furthermore, the energy from a BS must arrive at the MS within a guard interval period, which in OFDMA technology is referred to as a cyclic prefix interval.
The use of multiple antennas in wireless systems is not to be confused with the macrodiversity benefit in OFDMA systems supporting MBS. Macrodiversity in OFDMA systems supporting MBS may occur when using single or multiple antennas, but the use of multiple antennas is not a requirement for macrodiversity combining in OFDMA systems supporting MBS.
Macrodiversity may also require synchronization from a MBS server all the way to the BS transmitting within the single MBS zone. However, synchronization of BS operating in the single MBS zone may be difficult to achieve since the BS operating in the single MBS zone may be distributed across a large geographical area. Additionally, the BS may be connected using different transport mediums (e.g., E1/T1, OC3, satellite, and so forth). Therefore, the MBS data sent from the MBS server to the BS may arrive at the BS at different times due to different capabilities of the different transport mediums, media access control mechanisms, and so forth.
FIG. 1a illustrates a portion of a communications system 100 including a base station BS 105. BS 105 may have a coverage area 107. Operating in coverage area 107 may be mobile station ‘MS 1’ 110 and a mobile station ‘MS 2’ 112. BS 105 may be used to coordinate communications to and from MS 1 110 and MS 2 112. Since there is only one base station, transmissions to BS 105 and from BS 105 may not take advantage of macrodiversity.
FIG. 1b illustrates a portion of a communications system 150 including a base station ‘BS 1’ 155 and a base station ‘BS 2’ 157. BS 1 155 may have a coverage area 162, while BS 2 157 may have a coverage area 163. There may be some overlap in coverage areas 162 and 163 (shown as overlap area 164). Operating in coverage area 162 may be mobile station ‘MS 1’ 165 and in coverage area 163 may be mobile station ‘MS 2’ 167. Mobile station ‘MS 3’ 169 may be operating in both coverage areas 162 and 163 (i.e., MS 3 169 may be operating in overlap area 164).
BS 1 155 may transmit to and receive transmissions from MS 1 165 and MS 3 169, while BS 2 157 may transmit to and receive transmissions from MS 2 167 and MS 3 169. Furthermore, if BS 1 155 and BS 2 157 transmit the same data payload using the same format in a synchronized manner as described previously, MS 3 169 may be able to exploit macrodiversity by receiving transmissions from both BS 1 155 and BS 2 157.